380 GB in storage for multiple years of contents is really not much. I archive that amount every 2 months.
The real problem is serving all that content to the viewers, and the first bottleneck is usually the upload bandwidth.
I think the more interesting number would be to know how much data would it be to upload an average sized video to every viewer of it.
Using your example of a 15 MB video, serving that to 300.000 viewers means uploading roughly 4,5 TB data, plus some for technical data (TCP/IP and HTTP headers and such). For every (average) video! Now that’s a lot!
Fortunately PeerTube helps with that: viewers will automatically upload their downloaded chunks of the video to the others currently viewing it, so in the end the server needs somewhat less bandwidth usage.
Other than that, it would be the perfect place where channels could team up to host shared instances for themselves, or every channel their own one but with redundancy set up, so that their friend channels could also chip in with the bandwidth when needed.